Mesophotic coral bleaching associated with changes in thermocline depth

As global temperatures continue to rise, shallow coral reef bleaching has become more intense and widespread. Mesophotic coral ecosystems reside in deeper (30–150 m), cooler water and were thought to offer a refuge to shallow-water reefs. Studies now show that mesophotic coral ecosystems instead have limited connectivity with shallow corals but host diverse endemic communities. Given their extensive distribution and high biodiversity, understanding their susceptibility to warming oceans is imperative. In this multidisciplinary study of an atoll in the Chagos Archipelago in the central Indian Ocean, we show evidence of coral bleaching at 90 m, despite the absence of shallow-water bleaching. We also show that the bleaching was associated with sustained thermocline deepening driven by the Indian Ocean Dipole, which might be further enhanced by internal waves whose influence varied at a sub-atoll scale. Our results demonstrate the potential vulnerability of mesophotic coral ecosystems to thermal stress and highlight the need for oceanographic knowledge to predict bleaching susceptibility and heterogeneity.


Figure S2 :
Figure S2: Temperature with depth of the different ROV dives.Temperature was measured via a CTD mounted on the ROV.The different colours show the different ROV dives undertaken in November 2019.

Figure S3 :
Figure S3: Local temperature fluctuation in the Chagos Archipelago a) Temperature (°C) of the upper 300m between 2017 and 2021 in the Chagos Archipelago.The three isobaths indicate 22°C, 18°C and 14°C.b) corresponding IOD index.

Figure S4 :
Figure S4: Model-predicted bottom temperature variations at Ile Des Rats, March 2020.The isobaths (solid lines with the labels in meters) at Ile des Rats are overlayed with the model-predicted bottom temperature variations calculated over a 15-day cycle.

Figure
Figure S5: A) Model-predicted bottom temperature variations at Manta Alley, November 2019, calculated over a 15-days cycle.The temperature panel is overlaid with the isobaths (solid lines with labels).The track of the ROV dive is shown by magenta.B) In-situ water temperature recorded with the ROV (shown in panel A) in Manta Alley.The solid black line in panel B shows the background temperature profile recorded in November 2019.

Figure
Figure S6: A) Model predicted bottom temperature variations at Ile Des Rats, November 2019, calculated over a 15-days cycle.The temperature panel is overlaid with isobaths (solid lines with the labels in meters).B) In-situ water temperature recorded with the ROV (shown in panel A) in Manta Alley.The colours of the temperature records in panels A and B coincide.The solid black line in panel B shows the background temperature profile recorded in November 2019.

Figure S7 :
Figure S7: Predicted velocity time series overlaid with temperature isolines at Ile Des Rats, November 2019, for A) zonal and B) meridional.The position of the ADCP and thermistor chain are shown in Figure 8a.Data in the white rectangle C) and D) are used for comparison with similar in-situ data E) and F).

Figure S8 :
Figure S8: Temperature over depth and time during March 2020 when three moorings simultaneously monitored the temperature and currents in water of 65 m depth at a) west Ile Des Rats (IDR), b) east IDR and c) Manta Alley (MA); the instantaneous temperature at 45 m depth at each mooring, d), illustrates the significantly stronger cooling effect of the waves at both IDR sites compared to MA, whereby the waves were of larger amplitude and extended over a greater vertical range than in MA.

Table S2 -1: two-sided Permutational Analysis of variance (PERMANOVA) test results with
presence/absence data and Bray-Curtis similarity.999 permutations.Pairwise tests between the two studied sites, Ile Des Rats (IDR) and Manta Alley (MA) for each depth band at Egmont Atoll.P(perm): p-value

Table S2 -2: Average similarity between/within groups of two-sided PERMANOVA test, with
abundance data, square-root transformed, and Bray-Curtis similarity, 999 permutations.Pairwise tests between the two studied sites, Ile Des Rats (IDR) and Manta Alley (MA) for each depth band at Egmont Atoll.
Figure S1: Multidimensional scaling (MDS) plot on scleractinian coral abundance, square-root transformed and balanced data, with Bray Curtis similarity.A dummy variable was added to account for zero inflated data.IDR: Ile Des Rats; MA: Manta Alley.Depth in m.

Table S7 : 10%, 1% and 0.1% light optical depths for each deployment
. Depths are expressed in metres, with downward and upward light optical depths measured for each deployment undertaken in 2020 and 2022.Dates are expressed as month/year, each date is a different day.